Easy Money
Take the streetcar to Harumi-Cho. The earliest you can get there is during chapter 5. In the northeast corner of the map, just past the "suspicious foreigner," is a small dock. Summon a Volt Order Demon and use its Inspect ability while standing on the dock. A small light will appear -- check it. You are now able to use your Skill Demon's Transform ability to turn into the owner of Konnou-Ya. While walking around like this, Raidou will gain money every odd moon phase (1, 3, 5, etc.). Also, pressing the square button will cause a merchant to appear (as long as the area you're in is not dangerous). This saves a lot of backtracking and helps avoid the low item quantity caps.